The present invention relates to hydraulic power steering systems and more particularly relates to control valves for such systems.
Steering systems are known wherein a pair of double-acting hydraulic actuators are provided for steering a pair of wheels which are linked together such that steering movement of either wheel will be transferred to the other. Heretofore, these systems have normally been provided with controls for simultaneously effecting pressure-extension of one actuator and pressure-retraction of the other during steering operations. In these systems, the actuators are sized to develop the necessary torque to overcome the maximum steering resistance expected to be encountered, such as when operating in mud or ruts or when operating on a hillside, while the pump is sized to produce the flow necessary for effecting a relatively fast steering motion as when operating under normal conditions on even, dry terrain or on a road where steering resistance is low.
It will be appreciated, then, that these known systems are somewhat inefficient since the actuators have far too much torque-developing capacity for the times when the steering resistance is normal while the pump has far too much flow capacity for the times when the steering resistance is heavy. Thus, the system design is somewhat lacking in efficiency.
U.S. Pat. No. 3,249,174, issued on May 3, 1966, discloses a steering system wherein it is recognized that for a steering pump of a given size, the steering speed may be increased by routing the working fluid to only one of the pair of hydraulic steering actuators. This patented system thus may be designed to be more efficient than the known steering systems described hereinabove. However, the patented system has the drawback that the skill of the operator is relied upon since the operator must manually actuate a valve for routing pressure fluid to only one actuator when it is desired to speed up the steering response. While steering resistance is not mentioned in the patent, it is observed that unless a single one of the actuators has sufficient torque-developing capacity to effect steering motion when the steering resistance is high, the steering motion might stall if the operator diverts pressure fluid to only one actuator when the steering resistance is high.